Our long-term goal is to understand molecular mechanisms of HIV transcription and replication in infected host cells and to develop small-molecule chemical compounds that intervene the viral life cycle. The Specific Aims of this Project, as a component of theProgram Project Grant (PPG) application, stem from our discovery of bromodomains (BRDs) as acetyllysine binding domains, and from our recent findings that HIV transcriptional activation requires a specific interaction between the activated, lysine-acetylated H N transactivator protein Tat and the BRD of the transcriptional co-activator PCAF that have resulted from our collaborative investigation with the Verdin Laboratory (the Verdin Project). These new findings suggest a novel mechanism for potential therapeutic intervening of HIV gene expression and replication. In this Project, we propose to determine the detailed molecular, structural basis of Tat recognition by the PCAF BRD and to understand the differences of ligand selectivity between the PCAF and other structurally homologous BRDs. We will take advantage of our new knowledge of structural understanding of the molecular recognition emerging from our study and a parallel study of this important biological system by computational and bioinformatic analysis (the Weinstein Project), and employ an NMR structure-based approach to design small- molecule chemical inhibitors with high affinity and high selectivity to interfere the HIV Tat/PCAF BRD interaction. These lead compounds will be used as powerful tools to investigate the mechanisms of HIV transcriptional activation, and to validate the Tat/PCAF BRD interaction as a new therapeutic target for blocking HIV replication (the Verdin Project). These lead compounds will lay a foundation for therapeutic development for new treatment of HIV AIDS.